Donald Trump turned to polling data to justify his proposal to temporarily ban all Muslims from entering the United States.

"According to Pew Research, among others, there is great hatred towards Americans by large segments of the Muslim population," the Republican presidential candidate said in a Dec. 7 statement. "Most recently, a poll from the Center for Security Policy released data showing 25 percent of those polled agreed that violence against Americans here in the United States is justified as a part of the global jihad, and 51 percent of those polled agreed that Muslims in America should have the choice of being governed according to shariah."

There’s a lot packed into that quote, but we decided to focus on Trump’s claim that 25 percent of Muslims polled by the Center for Security Policy agree that global jihad justifies violence against Americans.

While the study Trump cited does exist, it’s not at all clear that it supports his argument that "there is great hatred towards Americans by large segments of the Muslim population." There are several important problems with the survey that call into question whether the results are representative of the entire U.S. Muslim population.

Trump’s poll

The Center for Security Policy, a hawkish think tank, surveyed 600 Muslims and released findings in June under the headline, "Poll of U.S. Muslims Reveals Ominous Levels Of Support For Islamic Supremacists’ Doctrine of Shariah, Jihad."

Among the findings is 25 percent of respondents agreed either slightly or strongly with the statement "Violence against Americans here in the United States can be justified as part of the global jihad." About 64 percent disagreed.

The first problem with the Center for Security policy poll has to do with methodology. It was an online, opt-in survey, which tend to produce less reliable samples because respondents choose to participate. In traditional polling methods, everyone in a population has a chance of being selected for the survey, meaning the results generally reflect the country’s demographics.

Numerous respected polling groups, like Survey Monkey, use opt-in surveys, said Christopher C. Hull, a former adjunct professor at Georgetown University who now works with the Center for Security Policy and answered an inquiry from PolitiFact on the group’s behalf. He added that the method is useful for reaching small populations, such as U.S. Muslims, who only make up about 1 percent of the population.

Hull did caution, however, that "one cannot extrapolate directly from an online, opt-in survey to the broader U.S. population."

One notable finding buried in the full survey data: It found that 23 percent of the U.S. Muslims surveyed said they are "not at all familiar with" the terrorist group known as the Islamic State, and 18 percent said they are not familiar with al-Qaida. Experts said it seems illogical that such a large percentage of American Muslims would not have knowledge of the two groups.

"The al-Qaida number seems entirely implausible and likely a canary in a coal mine as to the unrepresentativeness of this survey," said David Dutwin, executive vice president and chief methodologist at SSRS, a research firm.

There may be further problems with the poll, including the reality that many American Muslims are immigrants and not fluent in English, and that the survey asked leading questions with limited response choices, according to a critique by the Washington Post’s Philip Bump.

It’s also worth noting that the head of the Center for Security Policy, Frank Gaffney, has articulated a variety of theories about Muslim extremists that verge on conspiracy, such as the idea that the Muslim Brotherhood has infiltrated the United States government and the false narrative that President Barack Obama is Muslim.

Robert Oldendick, executive director of the Institute for Public Service and Policy Research at the University of South Carolina, reviewed the survey methodology made available by the Center for Security Policy but said he still didn’t have enough information to assess whether it’s a good or bad poll. The group has not disclosed how the surveyors targeted participants and what the response rate was.

"I would view these results very cautiously," Oldendick said. "It may be right, but it may not be. But the information to identify the quality of the sample is just not there."

Compared with Pew

Trump also mentioned research by the Pew Research Center as support for his proposal.

On a key point, the results of the Center for Security Policy study diverge from the results of a 2011 Pew Research study, a traditional telephone survey of 1,033 U.S. Muslims. The methodology of the Pew poll, and the organization’s reputation, is considered solid by public-opinion experts.

The Pew survey asked whether "suicide bombing/other violence against civilians is justified to defend Islam from its enemies." Just 1 percent said it is "often" justified, while 12 percent said it is "sometimes" or "rarely" justified. Eighty-one percent said it is "never" justified.

According to Pew in 2011, about 13 percent of American Muslims said they believe that violence in the name of Islam is justifiable. That’s half the rate of the Center for Security Policy finding of 25 percent.

The 2011 Pew study also found that a "significant minority" -- 21 percent -- of American Muslims feel that there is a great deal or a fair amount of support for extremism in the American Muslim community.

The Center for Security Policy survey results do not directly back up Trump’s proposal to stop Muslims from entering the United States, said Kellyanne Conway, president of the Polling Company, which conducted the survey on the center’s behalf.

"We did not -- nor would we -- ask whether the U.S. should ban all Muslims," she said.

The Center for Security Policy poll has touched a nerve because it "asked questions that other polling firms are refusing to ask," Hull said, defending the results.

"When people do not like the results of a poll, they attack the poll’s methodology and sponsor," he said. "This situation is no different."

Our ruling

Trump said that 25 percent of U.S. Muslims "agreed that violence against Americans here in the United States is justified as a part of the global jihad."

Trump is referring to a poll conducted by the Center for Security Policy. However, polling experts raise numerous questions about the validity of the poll’s results, including its "opt-in" methodology and the dubiously large percentages of respondents who said they were unaware of ISIS or al-Qaida. Moreover, an official with the Center for Security Policy cautioned against generalizing the poll results to the entire Muslim-American community.

Another survey, which experts consider credible, found levels about half as high as what the Center for Security Policy poll found.

Phone interview, University of South Carolina professor Robert Oldendick, Dec. 8, 2015

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